This question already has answers here:
Do temp variables slow down my program?
(5 answers)
Closed 5 years ago.
Let's say we have two functions:
int f();
int g();
I want to get the sum of f() and g().
First way:
int fRes = f();
int gRes = g();
int sum = fRes + gRes;
Second way:
int sum = f() + g();
Will be there any difference in performance in this two cases?
Same question for complex types instead of ints
EDIT
Do I understand right i should not worry about performance in such case (in each situation including frequently performed tasks) and use temporary variables to increase readability and to simplify the code ?
You can answer questions like this for yourself by compiling to assembly language (with optimization on, of course) and inspecting the output. If I flesh your example out to a complete, compilable program...
extern int f();
extern int g();
int direct()
{
return f() + g();
}
int indirect()
{
int F = f();
int G = g();
return F + G;
}
and compile it (g++ -S -O2 -fomit-frame-pointer -fno-exceptions test.cc; the last two switches eliminate a bunch of distractions from the output), I get this (further distractions deleted):
__Z8indirectv:
pushq %rbx
call __Z1fv
movl %eax, %ebx
call __Z1gv
addl %ebx, %eax
popq %rbx
ret
__Z6directv:
pushq %rbx
call __Z1fv
movl %eax, %ebx
call __Z1gv
addl %ebx, %eax
popq %rbx
ret
As you can see, the code generated for both functions is identical, so the answer to your question is no, there will be no performance difference. Now let's look at complex numbers -- same code, but s/int/std::complex<double>/g throughout and #include <complex> at the top; same compilation switches --
__Z8indirectv:
subq $72, %rsp
call __Z1fv
movsd %xmm0, (%rsp)
movsd %xmm1, 8(%rsp)
movq (%rsp), %rax
movq %rax, 48(%rsp)
movq 8(%rsp), %rax
movq %rax, 56(%rsp)
call __Z1gv
movsd %xmm0, (%rsp)
movsd %xmm1, 8(%rsp)
movq (%rsp), %rax
movq %rax, 32(%rsp)
movq 8(%rsp), %rax
movq %rax, 40(%rsp)
movsd 48(%rsp), %xmm0
addsd 32(%rsp), %xmm0
movsd 56(%rsp), %xmm1
addsd 40(%rsp), %xmm1
addq $72, %rsp
ret
__Z6directv:
subq $72, %rsp
call __Z1gv
movsd %xmm0, (%rsp)
movsd %xmm1, 8(%rsp)
movq (%rsp), %rax
movq %rax, 32(%rsp)
movq 8(%rsp), %rax
movq %rax, 40(%rsp)
call __Z1fv
movsd %xmm0, (%rsp)
movsd %xmm1, 8(%rsp)
movq (%rsp), %rax
movq %rax, 48(%rsp)
movq 8(%rsp), %rax
movq %rax, 56(%rsp)
movsd 48(%rsp), %xmm0
addsd 32(%rsp), %xmm0
movsd 56(%rsp), %xmm1
addsd 40(%rsp), %xmm1
addq $72, %rsp
ret
That's a lot more instructions and the compiler isn't doing a perfect optimization job, it looks like, but nonetheless the code generated for both functions is identical.
I think in the second way it is assigned to a temporary variable when the function returns a value anyway. However, it becomes somewhat significant when you need to use the values from f() and g() more than once case in which storing them to a variable instead of recalculating them each time can help.
If you have optimization turned off, there likely will be. If you have it turned on, they will likely result in identical code. This is especially true of you label the fRes and gRes as const.
Because it's legal for the compiler to elide the call to the copy constructor if fRes and gRes are complex types they will not differ in performance for complex types either.
Someone mentioned using fRes and gRes more than once. And of course, this is obviously potentially less optimal as you would have to call f() or g() more than once.
As you wrote it, there's only a subtle difference (which another answer addresses, that there's a sequence point in the one vs the other).
They would be different if you had done this instead:
int fRes;
int gRes;
fRes = f();
fRes = g();
int sum = fRes + gRes;
(Imagining that int as actually some other type with a non-trivial default constructor.)
In the case here, you invoke default constructors and then assignment operators, which is potentially more work.
It depends entirely on what optimizations the compiler performs. The two could compile to slightly different or exactly the same bytecode. Even if slightly different, you couldn't measure a statistically significant difference in time and space costs for those particular samples.
On my platform with full optimization turned on, a function returning the sum from both different cases compiled to exactly the same machine code.
The only minor difference between the two examples is that the first guarantees the order in which f() and g() are called, so in theory the second allows the compiler slightly more flexibility. Whether this ever makes a difference would depend on what f() and g() actually do and, perhaps, whether they can be inlined.
There is a slight difference between the two examples. In expression f() + g() there is no sequence point, whereas when the calls are made in different statements there are sequence points at the end of each statement.
The absence of a sequence point means the order these two functions are called is unspecified, they can be called in any order, which might help the compiler optimize it.
Related
so I get it that pre-increment is faster than post-increment as no copy of the value is made. But let's say I have this:
char * temp = "abc";
char c = 0;
Now if I want to assign 'a' to c and increment temp so that it now points to 'b' I would do it like this :
c = *temp++;
but pre-increment should be faster so i thought :
c = *temp;
++temp;
but it turns out *temp++ is faster according to my measurements.
Now I don't quite get it why and how, so if someone is willing to enlighten me, please do.
First, pre-increment is only potentially faster for the reason you state. But for basic types like pointers, in practice that's not the case, because the compiler can generate optimized code. Ref. Is there a performance difference between i++ and ++i in C++? for more details.
Second, to a decent optimizing compiler, there's no difference between the two alternatives you mentioned. It's very likely the same exact machine code will be generated for both cases (unless you disabled optimizations).
To illustrate this : on my compiler, the following code is generated when optimizations are disabled :
# c = *temp++;
movq temp(%rip), %rax
leaq 1(%rax), %rdx
movq %rdx, temp(%rip)
movzbl (%rax), %eax
movb %al, c(%rip)
# c = *temp;
movq temp(%rip), %rax
movzbl (%rax), %eax
movb %al, c(%rip)
# ++temp;
movq temp(%rip), %rax
addq $1, %rax
movq %rax, temp(%rip)
Notice there's an additional movq instruction in the latter, which could account for slower run time.
When enabling optimizations however, this turns into :
# c = *temp++;
movq temp(%rip), %rax
leaq 1(%rax), %rdx
movq %rdx, temp(%rip)
movzbl (%rax), %eax
movb %al, c(%rip)
# c = *temp;
# ++temp;
movq temp(%rip), %rax
movzbl (%rax), %edx
addq $1, %rax
movq %rax, temp(%rip)
movb %dl, c(%rip)
Other than a different order of the instructions, and the choice of using addq vs. leaq for the increment, there's no real difference between these two. If you do get (measurably) different performance between these two, then that's likely due to the specific cpu architecture (possibly a more optimal use of the pipeline eg.).
If a variable is not read from ever, it is obviously optimized out. However, the only store operation on that variable is the result of the only read operation of another variable. So, this second variable should also be optimized out. Why is this not being done?
int main() {
timeval a,b,c;
// First and only logical use of a
gettimeofday(&a,NULL);
// Junk function
foo();
// First and only logical use of b
gettimeofday(&b,NULL);
// This gets optimized out as c is never read from.
c.tv_sec = a.tv_sec - b.tv_sec;
//std::cout << c;
}
Aseembly (gcc 4.8.2 with -O3):
subq $40, %rsp
xorl %esi, %esi
movq %rsp, %rdi
call gettimeofday
call foo()
leaq 16(%rsp), %rdi
xorl %esi, %esi
call gettimeofday
xorl %eax, %eax
addq $40, %rsp
ret
subq $8, %rsp
Edit: The results are the same for using rand() .
There's no store operation! There are 2 calls to gettimeofday, yes, but that is a visible effect. And visible effects are precisely the things that may not be optimized away.
This sounds too much of a simple question to not be answered already somewhere, but I tried to look around and I couldn't find any simple answer. Take the following example:
class vec
{
double x;
double y;
};
inline void sum_x(vec & result, vec & a, vec & b)
{
result.x = a.x + b.x;
}
inline void sum(vec & result, vec & a, vec & b)
{
sum_x(result, a, b);
result.y = a.y + b.y;
}
What happens when I call sum and compile? Will both sum and sum_x be inlined, so that it will just translate to an inline assembly code to sum the two components?
This looks like a trivial example, but I am working with a vector class that has the dimensionality defined in a template, so iterating over operations on vectors looks a bit like this.
inline is just a hint to the compiler. Whether the compiler actually inlines the function or not is a different question. For gcc there is an always inline attribute to force this.
__attribute__((always_inline));
With always inlining you should achieve what you described (code generate as if it where written in one function).
However, with all the optimizations and transformations applied by compilers you can only be sure if you check the generated code (assembly)
Yes, inlining may be applied recursively.
The entire set of operations that you're performing here can be inlined at the call site.
Note that this has very little to do with your use of the inline keyword, which (other than its effect on the ODR — which can be very noticeable) is just a hint and nowadays mostly ignored for purposes of actually inlining. The functions will be inlined because your clever compiler can see that they are good candidates for it.
The only way you can actually tell whether it's doing this is to inspect the resulting assembly yourself.
It depends. inline is just a hint to the compiler that it might want to think about inlining that function. It's entirely possible for a compiler to inline both calls, but that's up to the implementation.
As an example, here's some prettified assembly output from GCC with and without those inlines of this simple program:
int main()
{
vec a;
vec b;
std::cin >> a.x;
std::cin >> a.y;
sum(b,a,a);
std::cout << b.x << b.y;
return 0;
}
With inlining:
main:
subq $40, %rsp
leaq 16(%rsp), %rsi
movl std::cin, %edi
call std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::_M_extract<double>(double&)
leaq 24(%rsp), %rsi
movl std::cin, %edi
call std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::_M_extract<double>(double&)
movsd 24(%rsp), %xmm0
movapd %xmm0, %xmm1
addsd %xmm0, %xmm1
movsd %xmm1, 8(%rsp)
movsd 16(%rsp), %xmm0
addsd %xmm0, %xmm0
movl std::cout, %edi
call std::basic_ostream<char, std::char_traits<char> >& std::basic_ostream<char, std::char_traits<char> >::_M_insert<double>(double)
movsd 8(%rsp), %xmm0
movq %rax, %rdi
call std::basic_ostream<char, std::char_traits<char> >& std::basic_ostream<char, std::char_traits<char> >::_M_insert<double>(double)
movl $0, %eax
addq $40, %rsp
ret
subq $8, %rsp
movl std::__ioinit, %edi
call std::ios_base::Init::Init()
movl $__dso_handle, %edx
movl std::__ioinit, %esi
movl std::ios_base::Init::~Init(), %edi
call __cxa_atexit
addq $8, %rsp
ret
Without:
sum_x(vec&, vec&, vec&):
movsd (%rsi), %xmm0
addsd (%rdx), %xmm0
movsd %xmm0, (%rdi)
ret
sum(vec&, vec&, vec&):
movsd (%rsi), %xmm0
addsd (%rdx), %xmm0
movsd %xmm0, (%rdi)
movsd 8(%rsi), %xmm0
addsd 8(%rdx), %xmm0
movsd %xmm0, 8(%rdi)
ret
main:
pushq %rbx
subq $48, %rsp
leaq 32(%rsp), %rsi
movl std::cin, %edi
call std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::_M_extract<double>(double&)
leaq 40(%rsp), %rsi
movl std::cin, %edi
call std::basic_istream<char, std::char_traits<char> >& std::basic_istream<char, std::char_traits<char> >::_M_extract<double>(double&)
leaq 32(%rsp), %rdx
movq %rdx, %rsi
leaq 16(%rsp), %rdi
call sum(vec&, vec&, vec&)
movq 24(%rsp), %rbx
movsd 16(%rsp), %xmm0
movl std::cout, %edi
call std::basic_ostream<char, std::char_traits<char> >& std::basic_ostream<char, std::char_traits<char> >::_M_insert<double>(double)
movq %rbx, 8(%rsp)
movsd 8(%rsp), %xmm0
movq %rax, %rdi
call std::basic_ostream<char, std::char_traits<char> >& std::basic_ostream<char, std::char_traits<char> >::_M_insert<double>(double)
movl $0, %eax
addq $48, %rsp
popq %rbx
ret
subq $8, %rsp
movl std::__ioinit, %edi
call std::ios_base::Init::Init()
movl $__dso_handle, %edx
movl std::__ioinit, %esi
movl std::ios_base::Init::~Init(), %edi
call __cxa_atexit
addq $8, %rsp
ret
As you can see, GCC inlined both functions when asked to.
If your assembly is a bit rusty, simply note that sum is present and called in the second version, but not in the first.
As mentioned, the inline keyword is just a hint. However, compilers do an amazing job here (even without your hints), and they do inline recursively.
If you're really interested in this stuff, I recommend learning a bit about compiler design. I've been studying it recently and it blew my mind what complex beasts our production-quality compilers are today.
About inlining, this is one of the things that compilers tend to do an extremely good job at. It was so by necessity, since if you look at how we write code in C++, we often write accessor functions (methods) just to do nothing more than return the value of a single variable. C++'s popularity hinged in large part on the idea that we can write this kind of code utilizing concepts like information hiding without being forced into creating software that is slower than its C-like equivalent, so you often found optimizers as early as the 90s doing a really good job at inlining (and recursively).
For this next part, it's somewhat speculative as I'm somewhat assuming that what I've been reading and studying about compiler design is applicable towards the production-quality compilers we're using today. Who knows exactly what kind of advanced tricks they're all applying?
... but I believe compilers typically inline code before you get to the kind of machine code level. This is because one of the keys to an optimizer is efficient instruction selection and register allocation. To do that, it needs to know all the memory (variables) the code is going to be working with inside a procedure. It wants that in a form that is somewhat abstract where specific registers haven't been chosen yet but are ready to be assigned. So inlining is usually done at this intermediate representation stage, before you get to the kind of assembly realm of specific machine instructions and registers, so that the compiler can gather up all that information before it does its magical optimizations. It might even apply some heuristics here to kind of 'try' inlining or unrolling away branches of code prior to actually doing it.
A lot of linkers can even inline code, and I'm not sure how that works. I think when they can do that, the object code is actually still in an intermediate representation form, still somewhat abstracted away from specific machine-level instructions and registers. Then the linker can still move that code between object files and inline it, deferring that code generation/optimization process until after.
The Problem: C++11 has made some changes to complex numbers so that real() and imag() can no longer be used and abused like member variables.
I have some code that I am converting over that passes real() and imag() to sincosf() by reference. It looks a little like this:
sincosf(/*...*/, &cplx.real(), &cplx.imag());
This now gives a error: lvalue required as unary '&' operand
which error was not received prior to c++11.
My Question: Is there an easy inline fix? or do I have to create temporary variables to get the result and then pass those to the complex number via setters?
Thanks
As T.C. mentions in the comments, the standard allows you to reinterpret_cast std::complex to your heart's content.
From N3337, §26.4/4 [complex.numbers]
If z is an lvalue expression of type cv std::complex<T> then:
— the expression reinterpret_cast<cv T(&)[2]>(z) shall be well-formed,
— reinterpret_cast<cv T(&)[2]>(z)[0] shall designate the real part of z, and
— reinterpret_cast<cv T(&)[2]>(z)[1] shall designate the imaginary part of z.
Moreover, if a is an expression of type cv std::complex<T>* and the expression a[i] is well-defined for an integer expression i, then:
— reinterpret_cast<cv T*>(a)[2*i] shall designate the real part of a[i], and
— reinterpret_cast<cv T*>(a)[2*i + 1] shall designate the imaginary part of a[i].
So make the following replacement in your code
sincosf(/*...*/,
&reinterpret_cast<T*>(&cplx)[0],
&reinterpret_cast<T*>(&cplx)[1]);
Just do
cplx = std::polar(1.0f, /*...*/);
Do not that declaring temporaries still is a good option and should be just as efficient as what you are previously doing, given a good optimizing compiler (say, gcc -O3).
See the resulting assembly from gcc -O3 here: https://goo.gl/uCPAa9
Using this code:
#include<complex>
std::complex<float> scf1(float x) {
float r = 0., i = 0.;
sincosf(x, &r, &i);
return std::complex<float>(r, i);
}
void scf2(std::complex<float>& cmp, float x) {
float r = 0., i = 0.;
sincosf(x, &r, &i);
cmp.real(r);
cmp.imag(i);
}
void scf3(std::complex<float>& cmp, float x) {
float r = 0., i = 0.;
sincosf(x, &cmp.real(), &cmp.imag());
}
Where scf2 is equivalent to your statement, you can see very similar assembly in the three cases.
scf1(float):
subq $24, %rsp
leaq 8(%rsp), %rsi
leaq 12(%rsp), %rdi
call sincosf
movss 12(%rsp), %xmm0
movss %xmm0, (%rsp)
movss 8(%rsp), %xmm0
movss %xmm0, 4(%rsp)
movq (%rsp), %xmm0
addq $24, %rsp
ret
scf2(std::complex<float>&, float):
pushq %rbx
movq %rdi, %rbx
subq $16, %rsp
leaq 8(%rsp), %rsi
leaq 12(%rsp), %rdi
call sincosf
movss 12(%rsp), %xmm0
movss %xmm0, (%rbx)
movss 8(%rsp), %xmm0
movss %xmm0, 4(%rbx)
addq $16, %rsp
popq %rbx
ret
scf3(std::complex<float>&, float):
pushq %rbx
movq %rdi, %rbx
subq $16, %rsp
leaq 8(%rsp), %rsi
leaq 12(%rsp), %rdi
call sincosf
movss 12(%rsp), %xmm0
movss %xmm0, (%rbx)
movss 8(%rsp), %xmm0
movss %xmm0, 4(%rbx)
addq $16, %rsp
popq %rbx
ret
While playing about with optimisation settings, I noticed an interesting phenomenon: functions taking a variable number of arguments (...) never seemed to get inlined. (Obviously this behavior is compiler-specific, but I've tested on a couple of different systems.)
For example, compiling the following small programm:
#include <stdarg.h>
#include <stdio.h>
static inline void test(const char *format, ...)
{
va_list ap;
va_start(ap, format);
vprintf(format, ap);
va_end(ap);
}
int main()
{
test("Hello %s\n", "world");
return 0;
}
will seemingly always result in a (possibly mangled) test symbol appearing in the resulting executable (tested with Clang and GCC in both C and C++ modes on MacOS and Linux). If one modifies the signature of test() to take a plain string which is passed to printf(), the function is inlined from -O1 upwards by both compilers as you'd expect.
I suspect this is to do with the voodoo magic used to implement varargs, but how exactly this is usually done is a mystery to me. Can anybody enlighten me as to how compilers typically implement vararg functions, and why this seemingly prevents inlining?
At least on x86-64, the passing of var_args is quite complex (due to passing arguments in registers). Other architectures may not be quite so complex, but it is rarely trivial. In particular, having a stack-frame or frame pointer to refer to when getting each argument may be required. These sort of rules may well stop the compiler from inlining the function.
The code for x86-64 includes pushing all the integer arguments, and 8 sse registers onto the stack.
This is the function from the original code compiled with Clang:
test: # #test
subq $200, %rsp
testb %al, %al
je .LBB1_2
# BB#1: # %entry
movaps %xmm0, 48(%rsp)
movaps %xmm1, 64(%rsp)
movaps %xmm2, 80(%rsp)
movaps %xmm3, 96(%rsp)
movaps %xmm4, 112(%rsp)
movaps %xmm5, 128(%rsp)
movaps %xmm6, 144(%rsp)
movaps %xmm7, 160(%rsp)
.LBB1_2: # %entry
movq %r9, 40(%rsp)
movq %r8, 32(%rsp)
movq %rcx, 24(%rsp)
movq %rdx, 16(%rsp)
movq %rsi, 8(%rsp)
leaq (%rsp), %rax
movq %rax, 192(%rsp)
leaq 208(%rsp), %rax
movq %rax, 184(%rsp)
movl $48, 180(%rsp)
movl $8, 176(%rsp)
movq stdout(%rip), %rdi
leaq 176(%rsp), %rdx
movl $.L.str, %esi
callq vfprintf
addq $200, %rsp
retq
and from gcc:
test.constprop.0:
.cfi_startproc
subq $216, %rsp
.cfi_def_cfa_offset 224
testb %al, %al
movq %rsi, 40(%rsp)
movq %rdx, 48(%rsp)
movq %rcx, 56(%rsp)
movq %r8, 64(%rsp)
movq %r9, 72(%rsp)
je .L2
movaps %xmm0, 80(%rsp)
movaps %xmm1, 96(%rsp)
movaps %xmm2, 112(%rsp)
movaps %xmm3, 128(%rsp)
movaps %xmm4, 144(%rsp)
movaps %xmm5, 160(%rsp)
movaps %xmm6, 176(%rsp)
movaps %xmm7, 192(%rsp)
.L2:
leaq 224(%rsp), %rax
leaq 8(%rsp), %rdx
movl $.LC0, %esi
movq stdout(%rip), %rdi
movq %rax, 16(%rsp)
leaq 32(%rsp), %rax
movl $8, 8(%rsp)
movl $48, 12(%rsp)
movq %rax, 24(%rsp)
call vfprintf
addq $216, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
In clang for x86, it is much simpler:
test: # #test
subl $28, %esp
leal 36(%esp), %eax
movl %eax, 24(%esp)
movl stdout, %ecx
movl %eax, 8(%esp)
movl %ecx, (%esp)
movl $.L.str, 4(%esp)
calll vfprintf
addl $28, %esp
retl
There's nothing really stopping any of the above code from being inlined as such, so it would appear that it is simply a policy decision on the compiler writer. Of course, for a call to something like printf, it's pretty meaningless to optimise away a call/return pair for the cost of the code expansion - after all, printf is NOT a small short function.
(A decent part of my work for most of the past year has been to implement printf in an OpenCL environment, so I know far more than most people will ever even look up about format specifiers and various other tricky parts of printf)
Edit: The OpenCL compiler we use WILL inline calls to var_args functions, so it is possible to implement such a thing. It won't do it for calls to printf, because it bloats the code very much, but by default, our compiler inlines EVERYTHING, all the time, no matter what it is... And it does work, but we found that having 2-3 copies of printf in the code makes it REALLY huge (with all sorts of other drawbacks, including final code generation taking a lot longer due to some bad choices of algorithms in the compiler backend), so we had to add code to STOP the compiler doing that...
The variable arguments implementation generally have the following algorithm: Take the first address from the stack which is after the format string, and while parsing the input format string use the value at the given position as the required datatype. Now increment the stack parsing pointer with the size of the required datatype, advance in the format string and use the value at the new position as the required datatype ... and so on.
Some values automatically get converted (ie: promoted) to "larger" types (and this is more or less implementation dependant) such as char or short gets promoted to int and float to double.
Certainly, you do not need a format string, but in this case you need to know the type of the arguments passed in (such as: all ints, or all doubles, or the first 3 ints, then 3 more doubles ..).
So this is the short theory.
Now, to the practice, as the comment from n.m. above shows, gcc does not inline functions which have variable argument handling. Possibly there are pretty complex operations going on while handling the variable arguments which would increase the size of the code to an un-optimal size so it is simply not worth inlining these functions.
EDIT:
After doing a quick test with VS2012 I don't seem to be able to convince the compiler to inline the function with the variable arguments.
Regardless of the combination of flags in the "Optimization" tab of the project there is always a call totest and there is always a test method. And indeed:
http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx
says that
Even with __forceinline, the compiler cannot inline code in all circumstances. The compiler cannot inline a function if:
...
The function has a variable argument list.
The point of inlining is that it reduces function call overhead.
But for varargs, there is very little to be gained in general.
Consider this code in the body of that function:
if (blah)
{
printf("%d", va_arg(vl, int));
}
else
{
printf("%s", va_arg(vl, char *));
}
How is the compiler supposed to inline it? Doing that requires the compiler to push everything on the stack in the correct order anyway, even though there isn't any function being called. The only thing that's optimized away is a call/ret instruction pair (and maybe pushing/popping ebp and whatnot). The memory manipulations cannot be optimized away, and the parameters cannot be passed in registers. So it's unlikely that you'll gain anything notable by inlining varargs.
I do not expect that it would ever be possible to inline a varargs function, except in the most trivial case.
A varargs function that had no arguments, or that did not access any of its arguments, or that accessed only the fixed arguments preceding the variable ones could be inlined by rewriting it as an equivalent function that did not use varargs. This is the trivial case.
A varargs function that accesses its variadic arguments does so by executing code generated by the va_start and va_arg macros, which rely on the arguments being laid out in memory in some way. A compiler that performed inlining simply to remove the overhead of a function call would still need to create the data structure to support those macros. A compiler that attempted to remove all the machinery of function call would have to analyse and optimise away those macros as well. And it would still fail if the variadic function made a call to another function passing va_list as an argument.
I do not see a feasible path for this second case.